1. Field of the Invention
The present invention relates to a proximity detecting device for detecting the presence of an object in a target area, comprising a low voltage power source, a circuit for generating electrical pulses and supplying them to a light emitting diode (LED) in order to emit light pulses directed to the target area, and a receiver for receiving the direct or reflected light pulses emitted from the LED towards the target area.
2. Description of Related Art
A proximity detecting device is used in particular for detecting the presence of an object, a human or a human body part and may be used to activate all kinds of different systems, such as alarm systems, automatic door opening systems, counting devices, automatic bathroom fittings, such as hand dryers, faucet devices, urinals, toilets, showers, soap dispensers, towel dispensers, wash fountains, etc. Known devices for detecting the presence of an object are either passive or active. A passive device detects the presence of an object, for example on the basis of its temperature or changes in light intensity on a passive ambient light sensor. An active device actively edits a signal, for example a light signal or an acoustic signal and detects corresponding signals which are usually reflected from an object present in the target area. The passive devices have the advantage of requiring generally a smaller amount of energy than the active devices since they do not have to emit signals. The active devices, however, have the advantage that also objects or persons which do not or almost do not move can be detected reliably, which is useful for example in the case of a person using a urinal.
Commercially available active devices generally use a 12 or 6 volt battery for delivering, through the intermediary of switching means, electrical pulses to a LED in order to emit light pulses having a duration equal to the duration of the electrical pulses.
In practice, when use is made of a 12 volt battery, a serial resistance, such as 10.OMEGA., is provided between the LED and the battery in order to reduce the voltage applied to the LED. The drawback of such a known device is that a substantial amount of energy supplied by the battery is lost in the serial resistance (10.OMEGA.). Indeed, in case the LED has a voltage drop of 1.5 volts, it can be deduced from the Ohm's law that the current through the LED is equal to 1.05A. The energy losses in the resistance of 10.OMEGA. are then 10.OMEGA.*(1.05A).sup.2 =11W for 12 volts in continuous mode.
A further drawback of this known active detecting device is that the LED must be energized each time for about 80 .mu.sec in order to emit light pulses having a sufficiently distant range. It is clear that the longer the pulse duration, the higher the amount of energy required for each light pulse (80 .mu.sec*1.05A=84 .mu.sec).